Luminescent probes are valuable reagents for the analysis and separation of molecules and cells and for the detection and quantification of other material. A very small number of luminescent molecules can be detected under optimal circumstances. Barak and Webb visualized fewer than 50 fluorescent lipid analogs associated with the LDL reception of cells using a SIT camera, J. Cell Biol. 90:595-604 (1981). Flow cytometry can be used to detect fewer than 10,000 fluorescein molecules associated with particles or certain cells (Muirhead, Horan and Poste, Bio/Technology 3:337-356 (1985). Some specific examples of application of fluorescent probes are (1) identification and separation of subpopulations of cells in a mixture of cells by the techniques of fluorescence flow cytometry, fluorescence-activated cell sorting and fluorescence microscopy; (2) determination of the concentration of a substance that binds to a second species (e.g., antigen-antibody reactions) in the technique of fluorescence immunoassay; (3) localization of substances in gels and other insoluble supports by the techniques of fluorescence staining. These techniques are described by Herzenberg et al., "Cellular Immunology," 3rd ed., chapt. 22; Blackwell Scientific Publications, 1978 (fluorescence-activated cell sorting); and by Goldman, "Fluorescence Antibody Methods" Academic Press, New York, 1968 (fluorescence microscopy and fluorescence staining); and in Applications of Fluorescence in the Biomedical Sciences, ed. Taylor et al., Alan Liss Inc., 1986.
When employing fluorescers for the above purposes, there are many constraints on the choice of the fluorescer. One constraint is the absorption and emission characteristics of the fluorescer, since many ligands, receptors, and materials in the sample under test, e.g. blood, urine, cerebrospinal fluid, will fluoresce and interfere with an accurate determination of the fluorescence of the fluorescent label. This phenomenon is called autofluorescence or background fluorescence. Another consideration is the ability to conjugate the fluorescer to ligands and receptors and other biological and non-biological materials and the effect of such conjugation on the fluorescer. In many situations, conjugation to another molecule may result in a substantial change in the fluorescent characteristics of the fluorescer and, in some cases, substantially destroy or reduce the quantum efficiency of the fluorescer. It is also possible that conjugation with the fluorescer will inactivate the function of the molecule that is labeled. A third consideration is the quantum efficiency of the fluorescer which should be high for sensitive detection. A fourth consideration is the light absorbing capability, or extinction coefficient, of the fluorescers, which should also be as large as possible. Also of concern is whether the fluorescent molecules will interact with each other when in close proximity, resulting in self-quenching. An additional concern is whether there is nonspecific binding of the fluorescer to other compounds or container walls, either by themselves or in conjunction with the compound to which the fluorescer is conjugated.
The applicability and value of the methods indicated above are closely tied to the availability of suitable fluorescent compounds. In particular, there is a need for fluorescent substances that emit in the longer wavelength visible region (yellow to near infrared), since excitation of these chromophores produces less autofluorescence and also multiple chromophores fluorescing at different wavelengths can be analyzed simultaneously if the full visible and near infrared regions of the spectrum can be utilized. Fluorescein, a widely used fluorescent compound, is a useful emitter in the green region although in certain immunoassays and cell analysis systems background autofluorescence generated by excitation at fluorescein absorption wavelengths limits the detection sensitivity. However, the conventional red fluorescent label rhodamine has proved to be less effective than fluorescein. Texas Red.RTM. is a useful labeling reagent that can be excited at 578 nm and fluoresces maximally at 610 nm.
Phycobiliproteins have made an important contribution because of their high extinction coefficient and high quantum yield. These chromophore-containing proteins can be covalently linked to many proteins and are used in fluorescence antibody assays in microscopy and flow cytometry. The phycobiliproteins have the disadvantages that (1) the protein labeling procedure is relatively complex; (2) the protein labeling efficiency is not usually high (typically an average of 0.5 phycobiliprotein molecules per protein); (3) the phycobiliprotein is a natural product and its preparation and purification is complex; (4) the phycobiliproteins are expensive; (5) there are at present no phycobiliproteins available as labeling reagents that fluoresce further to the red region of the spectrum than allophycocyanine, which fluoresces maximally at 680 nm; (6) the phycobiliproteins are relatively chemically unstable; (7) they photobleach relatively easily; (8) the phycobiliproteins are large proteins with molecular weights ranging from 33,000 to 240,000 and are larger than many materials that it is desirable to label, such as metabolites, drugs, hormones, derivatized nucleotides, and many proteins including antibodies. The latter disadvantage is of particular importance because antibodies, avidin, DNA-hybridization probes, hormones, and small molecules labeled with the large phycobiliproteins may not be able to bind to their targets because of steric limitations imposed by the size of the conjugated complex and the rate of binding of conjugates to targets is slow relative to low molecular weight conjugates.
Other techniques involving histology, cytology, immunoassays would also enjoy substantial benefits from the use of a fluorescer with a high quantum efficiency, absorption and emission characteristics at longer wavelengths, having simple means for conjugation and being substantially free of nonspecific interference.